1. Field of the Invention
The present invention relates to a sealing film for solar cell which mainly consists of ethylene-vinyl acetate copolymer and is used for a light receiving surface side of the solar cell. Furthermore, the present invention relates to a sealing film for a rear surface sealing film for a solar cell which mainly consists of ethylene-vinyl acetate copolymer.
2. Description of the Related Art
A solar cell has been paid attention as a device directly converting solar energy into electric energy from the viewpoints of effective use of natural resources and prevention of environment deterioration, and various solar cells have been developed.
In a solar cell, as described in FIG. 1, photovoltaic elements 4 made of silicon etc. are sealed by a light receiving surface sealing film 3A and a rear surface sealing film 3B made of EVA (ethylene-vinyl acetate copolymer) film between a glass plate as a transparent light receiving surface protection member 1 and a rear surface protection member 2 (backside covering material).
A solar cell is prepared by the following steps: the transparent light receiving surface protection member 1, the light receiving surface sealing film 3A, the photovoltaic elements 4, the rear surface sealing film 3B and the rear surface protection member 2 are superposed in this order and cross-linked the EVA by heating and pressing to unify the superposed product, thereby obtaining a solar cell.
When the solar cell is used under environment such as an outdoor environment in which the solar cell is exposed to high temperature and high humidity for a long period, water and moisture tend to permeate the solar cell. Conducting wires and electrodes of the solar cell are easily corroded with rust in the presence of the water and the moisture, resulting in reduction of a durability of the solar cell.
In order to improve the durability of the solar cell by preventing the conducting wires and the electrodes of the solar cell from corroding, the glass plate is used as the transparent light receiving surface protection member (see a patent document 1). However, even if the solar cell is sealed by using the glass plate, the durability of the solar cell cannot be sufficiently improved by preventing occurrence of rust.
In order to improve a photovoltaic performance of the solar cell, it is desired strongly to focus light to the photovoltaic elements by effectively collecting the light into the solar cell. From this viewpoint, it is preferred that the sealing film used for the solar cell has a high transparency, reflects and absorbs no incident light, and is capable of transmitting most of the light. For this reason, a colorless transparent EVA film having excellent water resistance is usually used as the sealing film for solar cell.
The EVA film is apt to generate acetic acid by the permeation of the water and the moisture at the high temperature with time, because the EVA comprises vinyl acetate as constituents. The acetic acid may accelerate the formation of rust by contacting with the wires and the electrodes of the solar cell. Therefore, in order to prevent highly the formation of rust of the solar cell, it is most effective to prevent the contact of the acetic acid with the conducting wires and the electrodes of the solar cell.
Document 2 discloses that a transparent EVA film which comprises an acid acceptor having an average particle diameter of not more than 5 μm in an amount of not more than 0.5% by weight is used as the sealing film for solar cell. The EVA film comprising the acid acceptor prevents the generation of the acid, thereby resulting in improvement the durability of the solar cell.    [patent document 1] JP-A-2000-174296    [patent document 2] JP-A-2005-29588